1. Technical Field
The present invention relates generally to electronic devices, and more particularly to a pulse width modulation system.
2. Description of the Prior Art
Pulse width modulation is a common method of amplifying an audio signal with high efficiency even at high levels of amplification. The technique is linear for a large signal and can be used to achieve a very high power gain. In pulse width modulation, the length of each pulse is indicative of the amplitude of the input signal over one cycle of a high-frequency ramp signal. The pulse can be compared to the ramp signal at a receiver to reconstruct the signal.
FIG. 1 illustrates a prior art pulse width modulation system 10. An input signal and a symmetric ramp signal are received at a pulse generator 12. The pulse generator 12 compares the two signals, outputting a voltage high during the periods when the input signal exceeds the ramp. For each cycle of the ramp signal, the pulse generator 12 will create a pulse of a width dependent upon the signal amplitude during that cycle. This is known as the duty cycle of the signal, and the length of an individual pulse is described as a percentage of this duty cycle.
The pulses are passed as a control signal to a switch 14. When the control signal is at a high voltage, the switch 14 closes a circuit including a high voltage source 15, a reconstruction filter 16, and a load 18. This applies the voltage across the load 18 to generate an output signal. When the control signal exhibits a low voltage, the switch 14 isolates the voltage source 15 from the load 18.
The reconstruction filter includes a number of inductors (e.g., 20 and 22) and capacitors (e.g., 24 and 26). These components store energy while the switch 14 is closed and release it as a residual voltage after the switch is opened. For example, the inductors 20 and 22 produce a reverse voltage according to the decrease in the current through the inductors after a certain time. To prevent this induced voltage from distorting the signal, a shunt diode 28 is provided on the circuit. The shunt diode 28 is positioned with its anode to ground such that it will conduct when the switch 14 is switched off. Thus, the shunt diode 28 allows the current flow generated by the inductors 20 and 22 a safe passage to ground when the filter 16 is isolated from the voltage source 15.
The system illustrated in FIG. 1 is effective for pulses having a high duty cycle. For high duty pulses, the amount of current passing through the inductors 20 and 22 is sufficiently large for its cessation to induce a significant reverse voltage. This reverse voltage overcomes a bias of the diode 28 to open the path to ground. When the duty cycle becomes small, however, the current flow through the inductors 20 and 22 is less, and hence the bias of the diode 28 is switched at much lower speed. In such a case, the induced voltage will artificially widen the voltage pulses provided to the load 18, distorting the output signal.
FIG. 2 illustrates a series of graphs showing the signal properties at various points within the prior art system. All of the graphs show the signal amplitudes against a progression of time. The first graph 40 illustrates a declining input signal 42 superimposed upon a cyclic ramp signal 44. The second graph 50 illustrates a drive signal that would be produced by the pulse generator 12 upon receiving the signals depicted in the first graph 40. The third graph 60 illustrates an output signal of the system. As the graphs indicate, the pulse width of the drive signal gets smaller as the distortion in the output signal grows larger. The output signal shows an increased pulse width when compared to the drive signal. The fourth graph 70 illustrates the results of this distortion. The graph compares the input signal 42 to a signal 72 recreated from the output of the pulse width modulator. As the graph illustrates, the recreated signal 72 diverges from the input signal 42 at low amplitudes.
FIG. 3 illustrates a second prior art system 80. An input signal and a ramp signal are received at a pulse generator 82. The pulse generator produces a drive signal that is provided to a first switch 84 as a control signal. The first switch 84 operates to provide a high voltage pulse from a voltage source 85 to a load circuit 86 comprising a reconstruction filter 88 and a load 90 in a manner similar to that described for FIG. 1 above.
The drive signal is also provided to a dead time generator 92. The dead time generator 92 produces an inverted drive signal containing a period of dead time after each pulse. This signal is provided as a shunt control signal to a second switch 94. The second, or shunt, switch 94 provides a path to ground from the load circuit 86. Because the second switch is controlled by an inversion of the drive signal controlling the first switch, the second switch should close just as the first switch opens. This creates a path to ground for the current produced by the reconstruction filter 88.
Unfortunately, the timing of the switches cannot be made exact. Such mistiming can lead to catastrophic system failure if the first switch 84 is not fully opened before the shunt switch 94 closes. The resulting short-circuit can destroy both switches. The dead time introduced in the dead time generator 92 prevents these timing failures. By leaving both switches off during a predetermined dead time, the timing issues on the switches can be avoided. Unfortunately, however, the dead time is itself a source of signal distortion. Accordingly, the system 80 of FIG. 3 provides little improvement over the system 10 of FIG. 1.